This invention relates to laser devices with wavelength and output power stabilization control and to methods of operating the same; more particularly, it relates primarily to such laser devices with at least two spectral narrowing elements, such as etalons, and to methods of operating them.
Recently, excimer lasers are attracting attention as the light source for exposure tools, such as steppers, for the production of integrated circuits. They are capable of generating light in a shorter ultraviolet wavelength region than conventional mercury light sources; for example, a KrF excimer laser generates light at the wavelength of 248 nm, which is far shorter than even the I line of the mercury at 365 nm. Thus, they are capable of achieving a degree of resolution in the production of integrated circuits, which has hitherto been considered impossible.
It is, however, difficult to build a refractive optical system with chromatic corrections in the extremely short wavelength range of the excimer lasers. On the other hand, the bandwidth of the excimer lasers are relatively large; for example, a KrF excimer laser produces light with a bandwidth in the order of 1 nm in the normal operation mode. Thus, reduction of the bandwidth of excimer lasers is necessary, if they are to be used as a light source for the optical system of wafer steppers, etc. As a result, excimer lasers having spectral narrowing elements have already been proposed, in which dispersive elements such as etalons, diffraction gratings, or prisms are disposed in the oscillator optical cavity in addition to the laser medium for generating the laser beam.
FIG. 1 shows the structure of a laser device having two intracavity etalons as spectral narrowing elements, which is described, for example, in T. J. McKee: "Spectral-narrowing technique for excimer laser oscillators," Can. J. Phys. vol 63, 214 (1985), pp. 214 through 219. The laser device comprises a laser medium (such as KrF excimer) 1, a totally reflecting mirror 2 disposed to one side of the medium 1, and a partially reflecting mirror (output coupler) 3 disposed to the other side of the medium 1; the medium 1 and the total and partial mirrors 2 and 3 constitute a laser oscillator (oscillator optical cavity). Fabry-Perot etalons 4 and 5 are disposed as spectral narrowing elements in the laser optical cavity, from which the laser beam 6 is emitted via the partial mirror 3. The surfaces of the etalons 4 and 5 are slightly tilted from a direction at right angles with the optical path of the laser, for the purpose of preventing total reflections. The etalons 4 and 5 consist of parallel glass plates partially silvered on their inner surfaces so that the incoming light is reflected back and forth between the two inner surfaces before being transmitted therethrough; the separation between the partially silvered inner surfaces of the coarse tuning etalon 4 is smaller than that of the fine tuning etalon 5. Due to the interference of the laser light passing therethrough, the etalons 4 and 5 act as band-pass filters with a plurality of transmission peaks, as explained below.
The operation of the laser device of FIG. 1 is as follows: When the laser medium 1 is excited by a voltage applied thereacross, coherent light is generated therein by repeated excitation and transition of the material (such as an excimer) of the laser medium 1. The light thus generated in the medium 1 is amplified in the optical cavity during the time in which it travels back and forth many times between the total and partial mirrors 2 and 3, to be ultimately emitted through the partial mirror 3 as a laser beam 6 at a predetermined output level.
In the case of an excimer, semiconductor, or dye laser, or certain kinds of solid state lasers, the oscillation frequency or wavelength width (i.e. bandwidth) of the laser generated by the medium 1 itself is relatively wide; as mentioned above, however, the bandwidth can be reduced by inserting spectral narrowing dispersive elements in the oscillator optical cavity. Thus, in the case of the device of FIG. 1, coarse and fine tuning etalons 4 and 5 are inserted in the cavity as spectral narrowing elements. The two etalons act essentially as band-pass filters. The fine tuning etalon 5 has high resolution (i.e. the width of the pass bands thereof is small) but includes a plurality of transmission peaks within the laser amplification band; the coarse tuning etalon 4 has a lower resolution and is used to select one of the transmission peaks of the fine tuning etalon 5. The detail of the spectral narrowing by the two etalons 4 and 5 is as follows.
FIGS. 2(a) and 2(b) show the spectral characteristics of the optical elements of the laser device shown in FIG. 1. Namely, FIG. 2 (a) shows the transmission characteristics of the coarse tuning etalon 4, wherein the transmission (i.e. the ratio of the intensity of light transmitted through the etalon) of the etalon 4 is shown as a function of the wavelength .lambda. plotted along the abscissa; 2(b) shows the transmission characteristics of the fine tuning etalon 5 in the same manner; (c) shows the laser gain profile of the laser medium 1 as a function of the wavelength .lambda.; (d) shows the output spectral characteristics of the laser beam 6 which has undergone the spectral narrowing via the intracavity etalons 4 and 5.
FIGS. 2(a) and 2(b), the wavelengths .lambda.m at the peaks of the transmission of the etalons 4 and 5 are given by: EQU .lambda.m=2.multidot.n.multidot.d.multidot.cos.theta. /m, (1)
wherein n is the refractive index of the material disposed between the two partially silvered surface of the etalon, d is the separation between the two silvered surfaces of the etalon, .theta. is the angle of incidence of light to the etalon, and m is an integer corresponding to the order of the etalon. The wavelengths .lambda.m at the peaks of the transmission of the coarse tuning etalon 4 are represented by .lambda.m, in FIG. 2(a); those of the fine tuning etalon 5 by .lambda.m.sub.2 in FIG. 2(b). (In both diagrams 2(a) and 2(b), only the wavelength of the transmission peak shown at the center in the figure is labelled with .lambda.m.sub.1 and .lambda.m.sub.1 ; Each transmission peak in FIG. 2(a) and 2(b) correspond to one of the values of the integer m.) The wavelengths .lambda.m at the transmission peaks in the neighborhood of laser oscillation band correspond to a value of about 10.sup.3 of the integer m; thus, the separations between the peaks along the abscissa .lambda. are substantially equal to each other in the figure. The wavelengths .lambda.m.sub.1 and .lambda.m.sub.2 at the peaks of the transmission of the coarse and fine tuning etalons 4 and 5, respectively, can be varied arbitrarily by changing the values of the parameters appearing in equation (1), i.e., the values of the reflectance n of the material between the silvered surfaces of the etalon, the separation d between the silvered surfaces of the etalon, and the angle of incidence .theta. of light to the surface of the etalon.
Further, the free spectral region FSR between adjacent peaks of the transmission of the etalons (which is represented by FSR.sub.1 and FSR.sub.2 in FIG. 2(a) and 2(b), respectively) is given by: EQU FSR=.lambda.m.sup.2 /2.multidot.n.multidot.d.multidot.cos.theta. =.lambda.m/m, (2)
while the half level width W.lambda. (which is represented by W.lambda..sub.1 and W.sub.2 in FIGS. 2(a) and 2(b), respectively) of each peak is given by: EQU W.lambda.=FSR/F, (3)
wherein F is a variable called finesse, whose value is determined by the parameters (such as the area) of the etalons; the value of the finesse F of the intracavity etalons is usually about 20 at most.
On the other hand, the laser gain profile of the laser medium 1, such as the excimer laser medium, is extended over a relatively wide wavelength band, as shown by a bell-shaped curve having an extended bottom at FIG. 2 (c). The laser gain profile represents the amplification gain characteristics of the medium 1: if no spectral narrowing elements such as etalons 4 and 5 are disposed in the oscillator optical cavity, the laser beam 6 amplified over the whole gain range (the wavelength range over which the gain profile is extended) is emitted via the partial mirror 3.
The spectral narrowing by the coarse tuning etalon 5 is effected as follows. Namely, the parameters of the etalon 5 are selected in such a manner that the following two conditions are satisfied: first, a wavelength .lambda.m.sub.1 at one of the peaks of the transmission coincides with an arbitrary predetermined wavelength .lambda..sub.0 set within the gain range (FIG. 2(c) shows the case where the setting wavelength .lambda..sub.0 is at the center of the gain region); second, the free spectral range FSR.sub.1 of the etalon 4 between its peaks of the transmission is wide enough to ensure that other peaks of the transmission do not fall within the gain range of the laser medium 1. Under these two conditions, only the light whose wavelength is within the pass band that includes the setting wavelength .lambda..sub.0 is transmitted through the etalon 5 and thus amplified in the oscillator optical cavity between the two mirrors 2 and 3.
However, the spectral narrowing by the insertion of one coarse tuning etalon 4 alone is limited. Namely, if the spectral narrowing is to be effected, the free spectral region FSR.sub.1 of the etalon 4 must be wide enough to ensure that only one of the wavelengths .lambda.m.sub.1 at the peaks of the transmission of the etalon 4 falls within the gain region of the laser medium 1. On the other hand, the value of the finesse F in equation (3) above is about 20 at most. Thus, the half level width W.lambda..sub.1 given by equation (3), which corresponds to the width of the pass band of the etalon 4, cannot be made smaller than a certain minimum value.
The bandwidth of the laser beam 6 can be further narrowed by the addition of the fine etalon 5 as follows. Namely, the the parameters of the etalon 5 are selected in such a manner that the following two conditions are satisfied: first, one of the wavelengths .lambda.m.sub.2 at the peaks of the transmission of the etalon 5 coincides with the setting wavelength .lambda..sub.0 ; and second, the free spectral region FSR.sub.2 between the peaks of the transmitting characteristics of the etalon 5 satisfies the inequality: EQU FSR.sub.2 .gtoreq.W.lambda..sub.1,
wherein W.lambda..sub.1 is the half width of the transmission peaks of the coarse tuning etalon 4. With the addition of the fine tuning etalon 5 satisfying the above two conditions, the spectral profile of the output laser beam 6 is narrowed and contracted around the setting wavelength .lambda..sub.0 as shown in FIG. 1(d).
Further spectral narrowing of the laser beam 6 can be accomplished by an insertion of still another fine tuning etalon, if desired, in a manner similar to the above.
By the way, when two etalons 4 and 5 are utilized, the design of the coarse tuning etalon 4 is made easier if the free spectral regions FSR.sub.1 and FSR.sub.2 of the etalons 4 and 5 satisfy approximately the following equation: EQU FSR.sub.1 =(k+1/2)FSR.sub.2, (4)
wherein k is an arbitrary integer (FIGS. 2(a) and 2(b) show the case where k is equal to 2). Namely, when the free spectral regions FSR.sub.1 and FSR.sub.2 satisfy approximately the above equation (4), spectral narrowing to the band around setting wavelength .lambda..sub.0 can be realized even if the free spectral region of the coarse tuning etalon 4 is narrower than the gain region of the laser medium 1.
As described above, the laser generated in the laser medium 1 having the amplification gain profile shown in FIG. 6(c) undergoes spectral narrowing via the etalons 4 and 5, through which the laser light goes back and forth many times during the amplification in the optical cavity; thus, the laser in the oscillator optical cavity is amplified in the narrow wavelength band centered around the setting, wavelength .lambda..sub.0, which coincides with the wavelengths .lambda.m.sub.1 and .lambda.m.sub.2 of one of the peaks of the transmission of the etalons 4 and 5. Since the light is transmitted through the etalons 4 and 5 a number of times, the bandwidth of the output laser beam 6, shown at FIG. 2 (d), is narrowed down to from 1/2 to 1/10 of the bandwidth which is obtained by the etalons 4 and 5 in the case where the laser light passes only once therethrough.
These laser devices, however, have problems with respect to the stability of frequency or wavelength of the laser beam. As discussed in the article by T. J. McKee cited above, the short term stability of the laser beam 6 can be enhanced by the improvement of the optical oscillator cavity or by reducing the incidence angle .theta. to the etalons 4 and 5; however, in the long term stability performance, the thermal effects, especially the wavelength shifts resulting from the heat generated by the laser beam 6 going through the etalons 4 and 5, present a major problem.
FIGS. 3(a) and 3(b) show the thermal shift of the peaks of the transmission of the coarse and the fine tuning etalons 4 and 5, respectively; FIG. 3(c) shows the shift in the wavelength band of the output laser beam 6 which results from the shifts in the transmission characteristics of the etalons 4 and 5 shown in FIGS. 3(a) and 3(b).
As shown in solid curves, the wavelengths .lambda.m.sub.1 and .lambda.m.sub.2 at the transmission peaks of the etalons 4 and 5, respectively, are aligned with the setting wavelength .lambda..sub.0 immediately after the lasing is started. When, however, the etalons 4 and 5 are deformed due to the heat generated by the laser light going through them, the separation d between the silvered surfaces of the etalons changes; as a result, the transmission peaks of the etalons 4 and 5 are shifted by widths .DELTA..lambda..sub.1 and .DELTA..lambda..sub.2, respectively, from the positions shown by solid curves to the positions shown in dotted curves in FIGS. 3(a) and 3(b), respectively. As easily derived from equation (1) above, the shift width .DELTA..lambda. of the transmission of the etalons is given by: EQU .DELTA..lambda.=(.lambda.m/d).multidot..DELTA.d, (5)
wherein .DELTA.d is the change in the separation d between the silvered surfaces of the etalons. Thus, the shift .DELTA..lambda. is positive (i.e. the transmission peaks are translated toward right in the figure) when the change .DELTA.d of the separation d is positive (i.e. the separation d increases); conversely, the shift .DELTA..lambda. is negative when the change .DELTA.d is negative.
As is apparent from equaion (5) above, the shift width .DELTA..lambda..sub.1 of the transmission peaks of the coarse tuning etalon 4 with a smaller separation d.sub.1 is larger than the shift width .DELTA..lambda..sub.2 of the transmission peaks of the fine tuning etalon 5 with a larger separation d.sub.2 ; namely: EQU .vertline..DELTA..lambda..sub.1 .vertline..ltoreq..vertline..DELTA..lambda..sub.2.vertline..
Thus, the peak transmission wavelength .lambda.m.sub.1 of the coarse tuning etalon 4 is deviated further from the setting wavelength .lambda..sub.0 than the peak transmission wavelength .lambda.m.sub.2 of the fine tuning etalon 5; thus, it follows that EQU .lambda.m.sub.1.noteq..lambda.m.sub.2.
As a result, the spectrum of the output laser beam 6 centered at the setting wavelength .lambda..sub.0, represented by the solid curve in FIG. 3 (c) is translated to the band, represented by the dotted curve therein, centered at the peak transmission wavelength .lambda.m.sub.2 of the fine tuning etalon 5. Further, the resultant transmission that results from the combination of the transmissions of the coarse and fine tuning etalons 4 and 5 decreases when the transmission peaks are translated from the positions shown by solid curves to those shown by dotted curves in FIGS. 3(a) and 3(b); consequently, the output power level at the central wavelength decreases by .DELTA.P as shown in FIG. 3(c).
In addition to the shift in the output wavelength of the laser beam 6 described above, oscillations at sidebands may take place when the shift width .DELTA..lambda..sub.1 of the wavelengths .lambda.m.sub.1 of the transmission peaks of the coarse tuning etalon 4 becomes large. Namely, as shown in dotted curves in FIG. 3(f), sideband outputs at wavelengths .lambda..sub.SA and .lambda..sub.SB are generated at positions corresponding to the bands at which one of the transmission peaks of the fine tuning etalon 5 is overlapped with a transmission band of the coarse tuning etalon 4, as shown in FIGS. 3(d) and 3(e).
With regard to the suppression of the generation of light at sideband wavelengths, V. Pol et al.: SPIE Vol. 633, Optical Microlithography V (1986) or Japanese Laid-Open Patent Application No. 63-228693, for example, proposes a laser device provided with a wavelength monitor mechanism which controls the intracavity etalons to suppress the osicillation at sideband wavelengths. The former proposes to utilize two monitor etalons in the wavelength monitor mechanism; and one of the two monitor etalons is used to detect the variation of the central wavelength of the laser beam, while the other is used to detect the sideband wavelengths, the intracavity etalons thereby being controlled on the basis of the detected wavelengths. On the other hand, the latter proposes to utilize an etalon in the wavelength monitor which has a free spectral region wider than those of the intracavity etalon; the wavelength of the laser beam is controlled on the basis of the intensity ratio of the central to the sideband wavelength.
These laser devices, however, have disadvantages. The disadvantage of the former is this: it utilizes two monitor etalons; thus, the structure of the device becomes complicated. On the other hand, the disadvantage of the latter is this: it controls the intracavity etalon on the basis of the intensity ratio of the sideband and central wavelength outputs of the laser beam; thus, the control becomes impossible when the sidebands disappear completely; in addition, since the amplification gain profile of the laser medium 1 varies with the change in its pressure or composition, the reference ratio on which the control is based must be adjusted accordingly. Further, in the case of the latter, since the free spectral region of the monitor etalon thereof is wide, it is impossible to determine the shift of the wavelength with high precision on the basis of the interference fringes formed by the monitor etalon. Namely, in the case where the sideband wavelengths .lambda..sub.SA and .lambda..sub.SB are generated due to the shift .DELTA..lambda..sub.1 in the transmission peaks of the coarse tuning etalon 4, as shown in FIGS. 3(f), the deviations of these sideband wavelengths .lambda..sub.SA and .lambda..sub.SB from the central wavelength is substantially greater than the width of the shift .DELTA..lambda..sub.2 of the central wavelength from the setting wavelength .lambda..sub.0. Thus, it is difficult to determine uniquely the sideband wavelengths .lambda..sub.SA and .lambda..sub.SB from the pattern of the interference fringes formed by the monitor etalon: if they are to be determined uniquely, the free spectral region of the monitor etalon must be set at a magnitude approximately equal to the width of the gain region of the laser medium. However, the gain region of the KrF excimer laser medium, for example, is not less than 400 pm; on the other hand, although the structure of the monitor etalon allows that the finesse F thereof be made greater than that of the intracavity etalons, the value of its finesse F is 50 at most. As a result, the half level width of the monitor etalon, which corresponds to its resolution of the wavelength detection, cannot be made smaller than about 8 pm. Thus, the detection of the central wavelength with high precision by means of the monitor etalon becomes impossible.
Thus, in the case of the former, the central and sideband wavelengths are detected separately by two monitor etalons. On the other hand, in the case of the latter, the ratio of the intensities at the central and sideband wavelengths are utilized in the wavelength control instead of the wavelengths themselves.
The disadvantages of conventional laser devices with spectral narrowing elements may be summarized as follows. First: the output power and the wavelength of the laser beam become unstable due to the thermal shifts of the transmission characteristics of the intracavity etalons when the laser oscillation is started or is started and stopped repeatedly; thus, laser devices of higher output power could not be put to practical use, since the intracavity etalons are subjected to heat generated by the high energy laser beam in the case of such devices. Second: in the case where the laser device comprises two or more intracavity etalons, sideband outputs may appear in the laser beam when the transmission peaks of the coarse tuning etalon is shifted due to its thermal deformation, etc.; since the wavelengths of the sidebands are deviated far away from the central setting wavelength of the laser beam, it is extremely important to detect and monitor the generation of these sidebands and suppress them as quickly as possible when they are detected. The conventional laser devices, however, are either incapable of sideband detection, or need a complicated structure for the detection of the sidebands.